1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an air cooled turbine airfoil having non-parallel pin fins.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine has a turbine section with a multiple stages of stationary vanes or nozzles and rotary blades or buckets exposed to extremely high temperature flow. The first stage vanes and blades are exposed to the highest temperature since the gas flow temperature progressively decreases through the turbine due to the extraction of energy. Especially in an industrial gas turbine engine, efficiency is the prime objective. In order to increase the efficiency of the engine, a higher gas flow temperature can be used in the turbine. However, the highest temperature that can be used depends upon the properties of the materials used in the turbine parts. For this reason, providing internal air cooling of the blades and vanes allows for a temperature higher than the material properties can withstand alone.
Another method of increasing the efficiency of the engine, for efficient use of the cooling air passing through the cooled airfoils is desired. Since the cooling air is generally bleed air from the compressor, maximizing the cooling effect while minimizing the amount of cooling air bled off from the compressor will increase the engine efficiency as well. Blade designers have proposed complex air cooling passages to maximize cooling efficiency while minimizing cooling volume. On a typical first stage turbine blade, the hottest surfaces occur at the airfoil leading edge, on the suction side immediately downstream from the leading edge, and on the pressure side of the airfoil at the trailing edge region. A showerhead arrangement is generally used to provide cooling for the leading edge of the airfoil. One problem blade designers are challenged with is that the hottest section on the suction side is also at a lower pressure than on the pressure side. A serpentine flow cooling circuit of the prior art that provides cooling for both the pressure side and the suction side will provide adequate cooling for the airfoil, but uses more cooling air that needed. Film cooling holes opening onto the pressure side and the suction side that are supplied with cooling air from the same cooling channel will both be discharging cooling air at the same pressure. Since the hot gas flow pressure on the suction side is lower than the pressure side, more cooling air will be discharged onto the suction side than is needed.
In a turbine airfoil with a serpentine flow cooling circuit, the cross sectional area of the passages must be sized in order than the airfoil walls will not be too thick. In many situations such as in open serpentine flow channels, some of the passages have cross sectional areas that are too large and result in low levels of heat transfer from the hot metal surface of the passage to the cooling air because the cooling air velocity is too low.
Turbine airfoils (which include blades and vanes) are typically cast as a single piece with the cooling passages cast within the airfoil. Ceramic cores having the cooling passage shape is used to form the airfoil. One problem with the prior art investment casting process that is used to produce a turbine airfoil is that the cooling passages within the airfoil have pin fins that are formed parallel to each other within the common passage or passages formed from a single ceramic core. Because of the die pulling direction in the die that is used to cast the ceramic core, the pin fins are limited to being in the pulling direction of the mold and thus are all parallel to each other. In some cast turbine airfoils, more than one ceramic core is sued. In this type, pin fins produced by one core are not required to be parallel to pin fins produced form a second core.
In a ceramic core used to form the inner cooling circuit of a turbine airfoil, the ceramic core includes pin fins forming projection that are arranged in parallel to each other and in a pulling direction of the mold used to cast the ceramic core. In the turbine airfoil of the present invention that has a 7-pass serpentine flow circuit; two ceramic cores are required to form the serpentine circuit. Each of the pin fin forming projections on a ceramic core must be formed parallel because of the pulling direction of the mold. For a two-core assembly, two different directions of pin fins can be formed because two molds are used to form the two cores.
In an investment casting process, there are minimum wall thicknesses that can be cast because of the viscosity of the molten metal and its capacity to flow through the mold and around the ceramic cores or through small holes or spaced. Also, with investment casting only a single metal or alloy can be poured into the mold. Thus, producing a single metallic piece of composite metal materials is not possible with this process.
Another problem with the prior art turbine blades produced using the investment casting process is that the blade root is cast without the fir tree configuration for mounting within the slots of the rotor disk. In this process, the blade is cast first and then the fir tree configuration is machined into the root portion. This adds further expense and complexity to the production of a turbine rotor blade.